05-19-2012, 08:23 PM
PF <> LoCA or Lateral CA, sorry Popo. As BC already indicated, many people posting on the internet about this, really see something that looks like some kind of bluish or purplish fringing, and immediately call it purple fringing. From a colour POV they may be right, as well as to the fact that it is some form of fringing.
Longitudinal Chromatic Aberration, LoCA, or Spherochromatism (which is the proper term), only occurs in the OOF areas, and consists of a magenta part in the OOF foreground area, and cyan in the background OOF area at high contrast transitions. Lateral CA generally is yellow and blue fringing, perpendicular to each other, at hich contrast transitions in th ein-focus area, and increases in intensity towards the edges of a photograph. Lateral CA may also show up in different colours.
Purple fringing is different in that it always is a bright purple, in both OOF and in-focus areas, that it doesn't have a complimentary colour in a different direction or other part of the OOF zones, and that it encircles the entire object at high contrast transitions, unlike Lateral or Longitudinal CA. This is why I stated that I like the samples Klaus provides, because with those one can be sure it is really PF, and not some form of true, optical CA.
As to film: film actually is less reflective than a sensor is: the average sensor reflects about 40% to 60% of the light reaching it, whereas film only reflects 2% to 3% - I do think that this is the main reason of higher intensity for PF in digital than on film. Internal reflections of the medium are therefore less too. The sensor has microlenses, and an AA-filter, whereas film only has emulsion and carrier surfaces as potential reflective areas, the most reflective of which likely is the carrier of the emulsion.
The relationship between LoCA and PF in my view is contentious at best. Spherical aberrations, which contribute to LoCA, are generally present in large aperture lenses. PF seems to occur more in tele lenses, and there is not necessarily a clear relationship between the two. However, I think this is really all subjective. It stays a subject of debate, for the simple reason that no one ever proved what it really is or what it really is caused by <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />. One can find a lot of debate and reasoning on the subject, but whereas it is (relatively) easy to make calculations for LoCA and Lateral CA, and make drawings to show how they appear and what they are, so far nobody has really been able to do so for proper PF. Generally, an extension of, e.g., a discussion of LoCA or Lateral CA includes PF, but it is never made clear in similar terms as to the how and what, essentially because nobody really knows (yet). And any of these extension essentially are opinions, not facts, AFAIAC.
Kind regards, Wim
Longitudinal Chromatic Aberration, LoCA, or Spherochromatism (which is the proper term), only occurs in the OOF areas, and consists of a magenta part in the OOF foreground area, and cyan in the background OOF area at high contrast transitions. Lateral CA generally is yellow and blue fringing, perpendicular to each other, at hich contrast transitions in th ein-focus area, and increases in intensity towards the edges of a photograph. Lateral CA may also show up in different colours.
Purple fringing is different in that it always is a bright purple, in both OOF and in-focus areas, that it doesn't have a complimentary colour in a different direction or other part of the OOF zones, and that it encircles the entire object at high contrast transitions, unlike Lateral or Longitudinal CA. This is why I stated that I like the samples Klaus provides, because with those one can be sure it is really PF, and not some form of true, optical CA.
As to film: film actually is less reflective than a sensor is: the average sensor reflects about 40% to 60% of the light reaching it, whereas film only reflects 2% to 3% - I do think that this is the main reason of higher intensity for PF in digital than on film. Internal reflections of the medium are therefore less too. The sensor has microlenses, and an AA-filter, whereas film only has emulsion and carrier surfaces as potential reflective areas, the most reflective of which likely is the carrier of the emulsion.
The relationship between LoCA and PF in my view is contentious at best. Spherical aberrations, which contribute to LoCA, are generally present in large aperture lenses. PF seems to occur more in tele lenses, and there is not necessarily a clear relationship between the two. However, I think this is really all subjective. It stays a subject of debate, for the simple reason that no one ever proved what it really is or what it really is caused by <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />. One can find a lot of debate and reasoning on the subject, but whereas it is (relatively) easy to make calculations for LoCA and Lateral CA, and make drawings to show how they appear and what they are, so far nobody has really been able to do so for proper PF. Generally, an extension of, e.g., a discussion of LoCA or Lateral CA includes PF, but it is never made clear in similar terms as to the how and what, essentially because nobody really knows (yet). And any of these extension essentially are opinions, not facts, AFAIAC.Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....